The present invention relates to additives for polyolefin resins, such as polyethylene and polypropylene.
It is known to use a wide variety of additives for polyolefin resins, such as stabilizers, binders, anti-static compounds, and the like, to improve or impart desirable properties.
For example, Pedrazetti et al., U.S. Pat. No. 4,708,979, teach that stabilizers may be advantageously incorporated into spherically polymerized polyolefins by the addition of certain tertiary amine compounds to an emulsion or dispersion of the desired stabilizers. The stabilizers enumerated include antioxidants, UV absorbers, and light stabilizers. Further, Caselli, U.S. Pat. No. 5,455,289, teaches a process for incorporating additives in a step following polymerization, and following deactivation of the aluminum alkyl catalyst, but prior to flashing of the monomer.
It is also known that certain classes of compounds can impart improved anti-static properties to polyolefin resins. Specifically, such compounds, sometimes referred to as internal chemical antistatic compounds, are classified as anionic, cationic (or just xe2x80x98ionicxe2x80x99) and non-ionic types. Ionic antistatic compounds are recommended for polar resin systems such as PVC. Generally, ionic antistatic compounds are not recommended for, e.g., polyethylene due to their low heat stabilities. Non-ionic antistatic compounds are organic compounds composed of hydrophilic and hydrophobic portions. The compound migrates to the substrate surface and, via hydrogen bonding with atmospheric water, creates a microscopic layer of water on the surface. Chemical antistatic compounds are therefore dependent upon atmospheric moisture for their mechanism to dissipate static electricity.
Three general types of antistatic compounds are used in polyethylene and polypropylene: glycerol monostearate (GMS), ethoxylated fatty acid amines (also referred to as hydroxyalkylamines), and diethanolamides.
Typical concentrations of anti-static compounds vary by the application for the resin, but can range from several hundred to few thousand parts per million by weight of the resin. A typical loading for providing antistatic properties is about 1000 parts per million (0.10%) by weight of the resin. Certain applications, e.g., food contact, require that upper limits (e.g., on the order of 1500 parts per million (0.15%) established by the Food and Drug Administration be observed. Lower limits are determined by the applicable antistatic performance standard for the particular application. For example, polyolefin films used for packaging propellants, explosives, or other materials potentially subject to ignition by static discharges must meet rigorous antistatic performance standards.
It has now been discovered that hydroxyalkylamines can, in addition to imparting antistatic properties, significantly improve the color, i.e., the whiteness, of polyolefin resins produced with specific types of catalysts. Improvements in whiteness of resins can significantly improve their marketability for many applications with high standards for appearance. Surprisingly, the discovered improvement in whiteness is not, however, observed in resins produced using other types of catalysts.
Types of polyolefins for which the present invention may advantageously employed include linear low density polyethylene, high density polyethylene, polypropylene, and their copolymers, provided that such polyolefins are produced utilizing chromium-based or metallocene catalysts.
Thus, in one aspect of the invention, the whiteness of polyolefin resins produced by chromium and metallocene catalysts is improved by the addition of an effective amount of a hydroxyalkylamine.
In yet another aspect of the invention, the whiteness number of resins produced by chromium and metallocene catalysts is increased by at least about 15, alternatively, at least about 20, by the addition of an effective amount of at least one hydroxyalkylamine.
In another aspect, the invention is a polyolefin composition having improved whiteness which includes a polyolefin resin produced using a catalyst selected from the group consisting of chromium-based catalysts and metallocene catalysts, and an amount of at least one hydroxyalkylamine which is lower than that required to impart antistatic properties.